1. Field of the Invention
The present invention relates to a solid-state image pickup device and, more particularly, to the control of an integral time of charges in the line sensor.
2. Description of the Related Art
Conventionally, electronic still cameras, video cameras, facsimiles and the like use a solid-state image pickup device composed of semiconductor photoelectric conversion elements. For example, charge-transfer elements such as CCD or BBD or MOS transistors are disposed in a matrix form or in a line to implement an image pickup device or an in-focus detecting sensor.
The problems with those devices are the leakage of charges at a time of transfer of signal charges and low light-detecting sensitivity.
To solve these problems a solid-state image pickup device has been developed which uses static induction transistors (hereinafter referred to as SITs). The SITs are one type of phototransistors which have functions of photoelectric conversion and amplification. In contrast with field effect transistors and junction transistors, the SITs have features of high input impedance, high-speed operation, nonsaturation characteristic, low noise and low power dissipation. Accordingly, by using the SITs as light-receiving elements it will be possible to implement a solid-state image pickup device of great sensitivity, high-speed response and wide dynamic range.
A conventional application of a solid-state image pickup device using the SITs to a line sensor will be described. There is a method in which, when the level cf light amount of the image of an object is small, an integral time is increased, while, when the level of light amount is large, the integral time is decreased, in order to keep an output level approximately constant independently of the level of light amount and thus extend the dynamic range for light amount. As an example, an automatic focus detecting device is disclosed in a Japanese laid-open Patent publication No. 57-64711 in which a photodiode (hereinafter referred to as photometric photodiode) adapted for controlling the integral time is provided adjacent to a CCD or MOS line sensor.
FIG. 1 shows a simplified circuit diagram of the device, and FIG. 2 shows a waveform diagram of signals associated with the device.
A photometric photodiode 2 is disposed adjacent to a photodiode array 1 having photodiodes arranged in a line. Photometric photodiode 2 has a length equal to the length of photodiode array 1. Output of photometric photodiode 2 is taken out through an output buffer 3. Outputs of photodiode array 1 are in parallel applied to a CCD shift register 5 via transfer gates 4 and then transferred through shift register 5 to be serially taken out via an output buffer 6. This arrangement forms a CCD line sensor.
The operation of the line sensor will be described with reference to the signal waveforms shown in FIG. 2. When an integral-start pulse is applied, voltages in photodiode array 1 and photometric photodiode 2 are initialized.
When photodiode array 1 and photometric photodiode 2 are irradiated with a light, photoelectric charges are integrated in the capacitance of photometric photodiode 2, and the output of photometric photodiode 2 decreases. The change .DELTA.V in output potential of photometric photodiode 2 is Qph/C where C stands for the capacitance and Qph represents the photoelectric charge integrated in the capacitance.
When the output potential of photometric photodiode 2 reaches a predetermined voltage Vref, a transfer pulse is produced. As a result, transfer gate 4 is turned on to terminate the integration, and the photoelectric charges stored in the capacitances of photodiode array 1 are transferred to CCD shift register 5 to be sequentially read out therefrom.
By changing the reference voltage Vref it is possible to change the integral time and hence the CCD output level. The reference voltage Vref may be set so that the CCD output level becomes a predetermined value. That is, in order to keep the exposure (=illuminance.times.integral time) constant, the reference voltage Vref is changed in accordance with illuminance.
An area sensor formed of SITs is described in a Japanese Patent application No. 61-176290 filed by the assignee of the present application. If only a part of the area sensor is driven, the area sensor can be used as a line sensor. The operation of the line sensor which is realized by picking up only one line component from the area sensor, will be described with reference to FIGS. 3A to 7.
FIG. 3A is a perspective view representing a structure of an SIT forming one pixel, and FIG. 3B is a circuit diagram of the line sensor.
As shown in FIG. 3A, on an n.sup.+ -type silicon substrate 11 acting as the drain of the SIT deposited is an n.sup.- -type epitaxial layer 12 forming a channel region. A shallow n.sup.+ -type source region 13 is formed into epitaxial layer 12. Source region 13 is surrounded with a p.sup.+ -type gate region 14 within epitaxial layer 12. A MOS capacitor 15 is formed over gate region 14 to which a gate pulse .phi.g is applied through the capacitor.
When gate region 14 is reverse-biased, a depletion layer is formed on the outside of the gate region. Hole-electron pairs are produced by a light incident on the depletion layer. The electrons are drained into source region 13 and drain region 11, while holes are stored in gate region 14. Thus, the potential of the gate is raised, and a current flowing between the drain and the source is modulated with a change in the gate voltage so that a light-dependent amplified current is obtained. Reference numeral 16 denotes an isolation region adapted for isolating the SIT from other SITs.
For description of the line sensor, FIG. 3B shows one row of pixels. Pixel SITs 20-1, 20-2, . . . 20-n each having the structure shown in FIG. 3A are arranged in a line to form a row 20 of pixel SITs. The SITs have their sources coupled to lines 21-1, 21-2, . . . 21-n which are coupled to an output line 23 via MOS transistors 22-1, 22-2, . . . 22-n acting as select switches in a readout circuit 22. Transistors 22-1, 22-2, . . . 22-n have their gates coupled to a scanning circuit 24 to receive scan pulses .phi.s1, .phi.s2, . . . .phi.sn therefrom.
The SITs have their gates coupled via capacitors 19-1, 19-2, . . . 19-n to a line 26 which in turn is coupled to a mix circuit 27. Mix circuit 27 applies gate pulse .phi.g to the gate of each SIT through line 26.
A reset circuit 28 comprises MOS transistors 28-1, 28-2, . . . 28-n whose gates are supplied with a reset pulse .phi.r and drains are coupled to lines 21-1, 21-2, . . . 21-n, respectively.
Mix circuit 27 sets the voltage of gate pulse .phi.g to Vrd when a drive pulse .phi.rd is at a "1" level, while to Vrs when .phi.rs is at a "1" level. When pulse .phi.g is applied to line 26, the pixels of SITs are enabled to read out stored charges, so that, when the pixels are sequentially selected by the scan pulses .phi.s1, .phi.s2, . . . .phi.sn, photoelectric signals are sequentially read out from the pixels.
FIG. 4 is a timing diagram of pulses for operating the line sensor, and FIG. 5 is a circuit diagram of one pixel of SIT and the associated peripheral circuit.
In FIG. 4, the pulse .phi.g has two types of a "1" level, i.e., Vrd and Vrs. More specifically, the pulse .phi.g has the readout level Vrd during a readout period t.sub.rd and the reset level Vrs during the following reset period t.sub.rs.
The scan pulses .phi.s1, .phi.s2, . . . .phi.sn sequentially go to a logic "1" level during the readout time t.sub.rd so that the pixels of SITs arrayed in the line are sequentially scanned.
The reset pulse .phi.rs goes to a "1" level for each of reset periods to reset the pixels of SITs from which signals have been read out.
In FIG. 5, Tp denotes an SIT, Tr a resetting MOSFET, Ts a selecting MOSFET, Cgd a parasitic capacitance across the gate and drain, Cgs a parasitic rapacitance across the gate and source, Cs a stray capacitance of a source line 21, and Ron an on resistance of selecting MOS transistor Ts.
Changes with time in the gate potential Vg and source potential Vs of the pixel Tp will be described with reference to FIG. 6, which illustrates the changes with time in the gate potential Vg and source potential Vs in the pixel SIT when the pulse .phi.s, the pulse .phi.g and the reset pulse .phi.r are applied to the pixel SIT. .phi.b stands for a built-in voltage between the gate and source which will be described later.
When the pulses .phi.g (=Vrs&gt;.phi.b) and .phi.r go to a "1" level (at a time t1), the source potential Vs is reset to ground potential and Vg becomes .phi.b (the built-in potential between the gate and source).
When the pulse .phi.g and .phi.r go to the ground level (at a time t2), the SIT is reverse-biased so that an integration is initiated. The gate voltage Vg (represented as Vg2 for time t2) is represented as follows: EQU Vg2=-(Cg/(Cg+Cj)).times.Vrs+.phi.b (1)
where Cj=Cgs+Cgd.
At a time t3 in an integration period, charges Qph produced by irradiation with a light are being stored in the gate capacitance (Cg+Cj). Qph is represented as follows: EQU Qph=Gl.times.A.times.P.times.t.sub.int =Gl.times.A.times.E (2)
where Gl stands for a generation rate (.mu.A/.mu.W), A an area (cm2) of a light-receiving surface, P an irradiance of light (.mu.W/cm2), t.sub.int an integration time (seconds), and E an exposure (E=P.times.t.sub.int).
By substituting equation (2) into equation (1) the gate voltage Vg3 at time t3 will be expressed as follows: EQU Vg3=-(Cg/(Cg+Cj)).times.Vrs+.phi.b+(Qph/(Cg+Cj)) (3)
At a time t4, since .phi.g=Vrd, the gate voltage Vg4 is represented by EQU Vg4=Vg3+(Cg/(Cg+Cj)).times.Vrd=(Cg/(Cg+Cj)).times.(Vrd-Vrs)+.phi.b+(Qph/(Cg +Cj)) (4)
When Vg4&gt;Vp (Vp is the gate-to-source potential difference at which the drain current of the SIT begins to flow and referred to as the pinch-off voltage), the drain current of the SIT flows so that the source line capacitance Cs is charged. This charging continues until the potential difference Vgs between the gate and source becomes Vp. Thus, the source potential Vs4 will be represented by EQU Vs4=(Cg/(Cg+Cj)).times.(Vrd-Vrs)+.phi.b+(Qph/(Cg+Cj))-Vp (5)
Since Vp&lt;.phi.b, a current hardly flows from the p.sup.+ -type gate to the n.sup.+ -type source of the SIT.
When the pulse .phi.s goes to a "1" level, (at a time t5), source line 21 is connected to a load resistor Rl via MOS transistor Ts (on resistance Ron). The output Vout (t) varies with time and represented as follows: EQU Vout (t)=(Rl/(Ron+Rl)).times.Vs(t) (6)
FIG. 7 illustrates the changes of gate voltage Vg, source voltage Vs and output Vout with time when the pulse .phi.s is at a "1" level.
When the pulse .phi.s goes to a "1" level, the p.sup.+ -type gate and n.sup.+ -type source of the SIT are biased in the forward direction with the result that a current flows through the pn junction diode, and signal charges stored in the gate capacitance flow into the source. Photoelectric signal charges are destroyed, and the gate voltage Vg and the source voltage Vs both decrease. The value of output Vout represented by equation (6) becomes smaller than a value obtained by substituting equation (5) into Vs(t) of equation (6).
As described above, the circuit of FIG. 3B can operate as a line sensor.
In addition to the sensor described above, the assignee of the present application has developed various improvements in the solid-state image pickup device and claimed an area sensor in a Japanese Patent application No. 61-277346.
FIG. 8 is a circuit diagram of the area sensor used as a line sensor, and FIG. 9 is a timing diagram therefor.
In the example of FIG. 3B, the integration time differs for each of the pixels. That is, the integration time is determined by the reset pulse .phi.rs and the scan pulse .phi.si (i=1.about.n). The reset pulse .phi.rs is commonly used for all the pixels. But, the scan pulses .phi.si for respective pixels are sequentially produced. In the circuit of FIG. 8, on the other hand, the same integration time is set to all the pixels. In FIG. 8, source voltages of all the SITs are transferred to the gate capacitances of transistors 31-i through transfer transistors 30-i at the same time. In addition, in the FIG. 3B circuit, a channel current of the SIT is taken out, while, in the FIG. 8 circuit, a source potential of an SIT which has been brought to a readout state is taken out.
Transfer switches 30-1, 30-2, . . . 30-n in a readout circuit 29 transfer source potentials of pixel SITs 20-1, 20-2, . . . 20-n to MOS transistors 31-1, 31-2, . . . 31-n through lines 21-1, 21-2, . . . 21-n. MOS transistors 31-1, 31-2, . . . 31-n are connected to a load resistor Rl via select switches 22-1, 22-2, . . . 22-n and a line 23, thereby forming a source follower circuit.
In operation, during a reset period trs a pulse .phi.g is set to Vrs in level, and pulses .phi.t and .phi.r are set to a "1" level, thereby resetting pixel SITs. The following operations during an integration period tint are the same as those of the SIT line sensor shown in FIGS. 3A through 7.
After the lapse of an integration period t.sub.int, when the pulse .phi.g is set to Vrd in level and the pulse .phi.t is set to a "1" level, the potentials (represented by equation (5)) on lines 21-1, 21-2, . . . 21-n are transferred to MOS transistors 31-1, 31-2, . . . 31-n. When pixel SITs 20-1, 20-2, . . . 20-n are selected by scan pulses .phi.s1, .phi.s2, . . . .phi.sn, the following output Vout is taken out from output line 23. EQU Vout=A.times.((Cg/(Cg+Cj)).times.(Vrd-Vrs)+Qph/(Cg+Cj)+.phi.b-Vp-Vt)(7)
where A is a voltage gain of the source follower circuit constituted by MOS transistors 31-1, 31-2, . . . 31-n, select switches 22-1, 22-2, . . . 22-n and load resistor Rl, and Vt is a threshold voltage of MOS transistors 31-1, 31-2, . . . 31-n.
This circuit arrangement permits a nondestructive read of the pixel SITs. If the pixel is not reset after the signal is read out from the pixel and the integration is continued to read out the signal from the pixel once more, it is necessary to change the timings of the pulses .phi.g and .phi.r. In order not to reset the pixel, the pulses .phi.g and .phi.r are set to a "0" level after the read period t.sub.rd.
Furthermore, another SIT sensor is shown in FIG. 10. In order for the SIT line sensor to have an integration-time control function, a photometric photodiode 41 is disposed adjacent to a pixel SIT 42. Reference numeral 43 denotes a readout circuit, 44 a scanning circuit, and 45 an output buffer circuit.
The above-described solid-state image pickup devices using SITs can overcome the drawbacks of the solid-state image pickup devices using MOS transistors, CCD, BBD or the like. The feature of the improved solid-state image pickup devices is that the integration time can be controlled in accordance with the amount of light by provision of a photometric photodiode adjacent to photodiode array and monitoring the potentials of the photodiode array.
However, with those devices, because the overall length of the pixel array is covered by one photometric photodiode an average measurement of light will be performed. Consequently, an accurate measurement of light cannot necessarily be obtained in some cases. For example, even if light is incident on only some of the pixels or on one pixel, the average photometric level is lower than the incident light level. Therefore, if the integration is stopped when the photometric output coincides with Vref, the output of the pixel onto which light is incident becomes larger than the predetermined level. Thus the integration time may not accurately be controlled.